SOLVED Low Volume Queen of Bone

[jplev22]

Hi All,

I decided to troubleshoot an old build that somehow could not figure put a few years ago. Queen of Bone 2 by rullywow.
The #2 side works without issue. In side #1 (right side) When it is engaged I can only get volume if gain is above 0. Also, the volume pot increases volume up to about 1 oclock and then comes down. Max volume is slightly bellow unity. Plenty of volume on side #2.

Here I what I did:

- Checked voltage at the opamp, comparing both sides, they seem quite similar.
-Diodes switches are both off (clean boost mode)
- Traced the audio circuit with an osciloscope. It seems the gain & tone circuits function as intended. (1Khz signal) If volume is at 0, the audio signal is strong at the volume pot input pin.
-I removed both output caps and resistor. I could then control the output signal with the volume pot. I then thought, it must be a bad cap.
-Installed new output Caps (C9 & C10) and resistor.
-Disconected switching daughterboard to mesure only #1 circuit. Same behavious, volume goes down after about 1oclock.
-Replaced opamp
-Replaced charge pump
-Replaced volume pot.

This is puzzling...

Thanks for any insight!

 

[jplev22]

Here are photos of the board. Tried a few things this morning, still get same result....

 

[zgrav]

"I can only get volume if gain is above 0."
could be a bad gain pot that loses connectivity when turned down all the way.

 

[jplev22]

I check the gain pot and seemed A O.K.
I traced the circuit once again making sure to compare both circuit. (This is essentially two bluesbreaker circuits in the same box)

Both sides are identical all the way to the volume pot.

I tested with a 4mV audio signal at the input.
I get 5.2mV at the volume pot with the gain knob all the way down. 23.4mV with gain all the way up.

I tested with a coupling cap connected directly at the volume pot. I get full volume if I do it this way. See photo.

I'm thinking of going off-board for the output caps and 1M grounding resistor... Maybe a bad trace grounding over about 5mV...

 

[jplev22]

Well I apparently did all that nonsense troubleshooting and the problem is on the switching board. If I take the output off circuit #1 (red wire); I get full volume, good sound. Connected to the switch, same problem.

Bad switch, maybe?

 

[jplev22]

Working now.